The present invention relates to an optical switch, and in particular, to an optical switch which electrically controls the direction in which an optical signal proceeds, and to a method of operating the optical switch, and a method of designing the optical switch.
Conventionally, in the field of optical communications, optical switches which electrically control the direction in which the optical signal proceeds have been used. One example of such an optical switch is a reversed xcex94xcex2 direction coupler optical switch.
The structure of a reversed xcex94xcex2 direction coupler optical switch will be discussed hereinafter with reference to FIG. 7. FIG. 7 is a plan view which illustrates the main structural portions, i.e., only the waveguide portions and electrode portions, of a reversed xcex94xcex2 direction coupler optical switch 100 (hereinafter xe2x80x9cswitch 100xe2x80x9d).
The optical switch 100 comprises a linear first waveguide 102, a linear second waveguide 104 which is disposed parallel to the waveguide 102, a linear first electrode 106 and a linear second electrode 108 which are disposed on the first waveguide 102 so as to be spaced apart from each other, and a linear third electrode 110 and linear fourth electrode 112 which are disposed on the second waveguide 104 so as to be spaced apart from each other. The first electrode 106 and the third electrode 110 oppose one another, whereas the second electrode 108 and the fourth electrode 112 oppose one another.
A coupling arises between the respective propagation modes of the first waveguide 102 and the second waveguide 104. While the polarity (e.g., positive) of the voltage with respect to a given reference potential, which voltage is applied to the first electrode 106 and the fourth electrode 112, and the polarity (e.g., negative) of the voltage with respect to a given reference potential, which voltage is applied to the second electrode 108 and the third electrode 110 are reversed, by controlling the magnitudes of these voltages, for example, an optical signal inputted from an input port P101 is outputted from either an output port P103 (a so-called xe2x80x9cbar statexe2x80x9d) or an output port P104 (a so-called xe2x80x9ccross statexe2x80x9d).
The range of values of the voltages which are applied to the electrodes and which are needed in order to bring the optical switch 100 into the cross state is broad. (Hereinafter, these values are referred to as xe2x80x9ccross voltage valuesxe2x80x9d.) Namely, the operational range of the cross state is broad. Therefore, the applied voltages can be easily set to cross voltages, and as a result, switching to the cross state can be carried out stably.
However, the range of values of the voltages which are applied to the electrodes and which are needed in order to bring the optical switch 100 into the bar state is narrow. (Hereinafter, these values are referred to as xe2x80x9cbar voltage valuesxe2x80x9d.) Namely, the operational range of the bar state is narrow. Therefore, it is difficult to set the applied voltages to bar voltages. As a result, even if attempts are made to switch the optical switch 100 to the bar state, a problem arises in that the optical signal leaks in the cross direction.
An example of another optical switch is disclosed in Applied Physics Letters, Vol. 35, No. 10, November 1979, pp. 748-750. In accordance with the structure of this optical switch (which is referred to hereinafter as xe2x80x9cthe second optical switchxe2x80x9d), a first waveguide is a linear waveguide, whereas a second waveguide is a gently curved waveguide which is convex toward the first waveguide. Electrodes are provided for the first waveguide and the second waveguide. Switching is carried out by independently controlling the voltages which are applied to these electrodes.
The range of bar voltage values for the second optical switch is broad. Namely, the operational range of the bar state is broad. Accordingly, it is easy to set the applied voltages to bar voltages. As a result, switching to the bar state can be carried out stably.
However, the range of cross voltage values in the second optical switch is narrow. Namely, the operational range of the cross state is narrow. As a result, it is difficult to set the applied voltages to cross voltages. As a result, even if attempts are made to switch to the cross state, a problem arises in that the optical signal leaks in the bar direction.
Thus, an optical switch in which the operational range of the bar state is broad and the operational range of the cross state is broad is desired. Further, a method of operation of such an optical switch and a method of designing such an optical switch is desired.
In order to achieve the above objects, the present invention provides an optical switch for outputting an optical signal in different states, comprising: (a) first and second waveguides each including a propagation mode, the waveguides having a plurality of mode coupling regions permitting coupling between propagation modes and a plurality of non mode coupling regions where substantially no coupling arises between propagation modes of the waveguides, the mode coupling regions being opposing portions of the waveguides, and the non mode coupling regions also being opposing portions of the waveguides, wherein the opposing portions of the mode coupling regions are nearer to one another relative to the opposing portions of the non mode coupling regions; and (b) a plurality of electrodes spaced apart from one another, wherein the electrodes are disposed along at least the first waveguide.
In accordance with this structure, as will be explained later, operational ranges of the bar state and the cross state are made wide by appropriately adjusting the voltages of the respective electrodes. Accordingly, switching to the bar state or to the cross state is stable.
In this invention, preferably, the electrodes are disposed along the first waveguide and the second waveguide, with the electrodes disposed along the first waveguide opposing, in a one-to-one relationship, the electrodes disposed along the second waveguide. Or, an electrode is provided at mode coupling regions, and no electrode is provided at non mode coupling regions.
When operating the above optical switch, preferably, when the electrodes are disposed along only one of the first and second waveguides, voltages are applied independently to the respective electrodes, and the polarities of the voltages are successively reversed, with respect to a given reference potential, in the order in which the electrodes are arranged. Or, when electrodes are disposed along the first and second waveguides, voltages are applied independently to the respective electrodes, and the polarities of the voltages are successively reversed, with respect to a given reference potential, in the order in which the electrodes are disposed in a line. Further, absolute values of differences between the reference potential and a potential of each of the electrodes disposed so as to oppose one another along the first and second waveguides are equal.
In accordance with such methods, the operational ranges of the bar state and the cross state are both broad. Accordingly, switching to the bar state or to the cross state is stable.
In the present invention, preferably, at least one of the waveguides includes a periphery having at least one curve. Or, the first and second waveguides each include a periphery having at least one curve with a convex section, with the convex section of the curve of each waveguide opposing the convex section of the curve of the other waveguide.
The structures of the first and second waveguides are substantially the same as the structure of a waveguide of a conventionally known optical wavelength filter. Accordingly, it is easy to design the waveguide. Further, the unit curve may be, for example, a substantially cycloid curve, or may be a substantial sine curve.
Further, in the present invention, preferably, the waveguides substantially mirror one another.
In order to achieve the above-described objects, the present invention also provides a method for designing an optical switch from an optical wavelength filter having at least one electrode and first and second waveguides with a plurality of portions opposing one another across a space, with the space between one opposing portion being less relative to the space between another opposing portion, at least one of the waveguides having a section with a relief-type grating structure, and another section with a relief-type grating structure reversed relative to the other grating structure for optical-wave splitting, the method comprising the steps of: (a) for the section with a relief-type grating structure that is not reversed, replacing the grating structure with an electrode; and (b) reversing a voltage potential applied to each electrode in accordance with the reversed relief-type grating structure.
Or, in designing an optical switch from an optical wavelength filter, the following method may be used.
A method for designing an optical switch from a direction coupler type optical wavelength filter having a first waveguide and a second waveguide, each waveguide having a non mode coupling region with a length, the length being different in each waveguide, the method comprising the steps of: (a) modifying the filter to have first and second waveguides of substantially the same length in the non mode coupling regions; (b) placing electrodes along at least one of the waveguides; and (c) independently controlling voltages applied to the electrodes.